Thank you very much for the anecdote, Franz.
Perhaps you missed previous messages on this subject, but apparently there are no uranium decay daughters (Rn-222, etc) in "new" or relatively recently grown coral (as one might expect on or near the surface of atolls), since uranium is selectively incorporated into the material, and therefore not in equilibrium with its progeny :
"U forms complexes with Ca in seawater, which explains the association with coral calcium. Elements like Th and Pr precipitate out of the water column because they do not form complexes."
So one would only expect to see uranium decay daughters in "old" coral.
As others have also said, that's how the age of coral is determined in the laboratory.
Kai Kaletsch provided the link to http://horizon.ucsd.edu/palmyra/cobb2002.pdf wherein Chapter 4 on "U/Th-dating living and young fossil corals from the central tropical Pacific" is certainly worth a look (Thanks Kai !)
Jaro
^^^^^^^^^^^^^^^^^^
Qualitatively I can tell you that we were not able to detect Rn-222 in indoor air - there is simply none, because corals do not contain uranium and therefore no Ra-226. So, what about this story?